Graphs and Combinatorics DOI 10.1007/s00373-016-1681-3 ORIGINAL PAPER Binomial partial Steiner triple systems containing complete graphs Małgorzata Pra˙zmowska1 · Krzysztof Pra˙zmowski2 Received: 1 December 2014 / Revised: 25 January 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract We propose a new approach to studies on partial Steiner triple systems con- sisting in determining complete graphs contained in them. We establish the structure which complete graphs yield in a minimal PSTS that contains them. As a by-product we introduce the notion of a binomial PSTS as a configuration with parameters of a minimal PSTS with a complete subgraph. A representation of binomial PSTS with at least a given number of its maximal complete subgraphs is given in terms of systems of perspectives. Finally, we prove that for each admissible integer there is a binomial PSTS with this number of maximal complete subgraphs. Keywords Binomial configuration · Generalized Desargues configuration · Complete graph Mathematics Subject Classification 05B30 · 05C51 · 05B40 1 Introduction In the paper we investigate the structure which (may) yield complete graphs contained in a (partial) Steiner triple system (in short: in a PSTS). Our problem is, in fact, a particular instance of a general question, investigated in the literature, which STS’s B Małgorzata Pra˙zmowska [email protected] Krzysztof Pra˙zmowski [email protected] 1 Faculty of Mathematics and Informatics, University of Białystok, ul. Ciołkowskiego 1M, 15-245 Białystok, Poland 2 Institute of Mathematics, University of Białystok, ul. Ciołkowskiego 1M, 15-245 Białystok, Poland 123 Graphs and Combinatorics (more generally: which PSTS’s) contain/do not contain a configuration of a prescribed type. Here belongs, e.g. the problem to determine Pasch-free configurations in some definite classes of (P)STS’s [13], or to characterize STS’s which do not contain the mitre configuration (e.g.: [7,23]). It resembles also (and, in a sense, generalizes) the problem to determine triangle-free (e.g. [1,10]), quadrangle-free (e.g. [3]), and other K-free (e.g. [6,16]) configurations, where K is a fixed class of configurations (of a special importance). In our case configurations in question are complete graphs and so-called binomial configurations. In essence, in what follows, speaking about a (complete) graph G contained in a configuration M we always assume that G is freely contained in M i.e. it is not merely a subgraph of the collinearity (point-adjacency) graph of M but also distinct edges of G lie on distinct lines (so called sides)ofM, and sides do not intersect outside G. Note that following this terminology a complete quadrangle on a projective plane P is not a K4-graph contained in P. There are two main problems that this theory starts with. Firstly: what are the minimal parameters of a PSTS necessary to contain a Kn-graph and does there exist a PSTS with these parameters which contains/does not contain a Kn-graph. It turns out that a minimal (with respect to its size) PSTS that contains Kn is a so called (v ) v = m = − binomial configuration i.e. a r b3 -configuration such that 2 , r m 2, and = m = + b 3 for a positive integer m ( n 1) (Prop. 3.2). We say, in short, that this PSTS is a minimal configuration which contains Kn. Several classes of binomial configurations were introduced and studied in the liter- ature (see [19, combinatorial Grassmannians], [17, combinatorial Veronesians], [18, multiveblen configurations], [20,21]). The above characterization of the parameters of a binomial configuration gives a good motivation and justification for investigating this class from ‘a general perspective’. In most of the binomial configurations already defined in the literature a suitable complete graph can be found. As the best known example of a binomial configuration we can quote generalized Desargues configura- tion(see[4,5,12]). Then the answer to the ‘dual’ question what is the maximal size of n n M a complete graph that a binomial 2 n−2 3 3 -configuration may contain easily − follows: this size is n 1. In this case we say that Kn is a maximal (complete) sub- M n n graph of . However, there are 2 n−2 3 3 -configurations that do not contain any Kn−1-graph. The simplest example can be found for n = 5: it is known that there are 103-configurations without K4.(cf.[2,15]). n+1 n+1 The second problem is read as follows: provided a 2 n−1 3 3 -configuration contains a Kn, what is the possible number of Kn-graphs contained in M and what is the structure they yield. It turns out that the maximal number of such Kn-subgraphs is n +1 (Prop. 3.9). Binomial configurations with the maximal number of Kn-subgraphs are exactly the generalized Desargues configurations. This fact points out once more a special position of the class of generalized Desargues configurations within the class of binomial configurations. n+1 n+1 As we already mentioned, the problem whether a given binomial 2 n−1 3 3 - configuration M contains a complete Kn-graph is slightly similar to the problem if a PSTS contains a Pasch configuration. Indeed, M contains such a graph iff it contains 123 Graphs and Combinatorics a binomial n n -configuration (Prop. 3.3,Cor.3.4). So, (binomial) configu- 2 n−2 3 3 n+1 rations (of size 2 ) without Kn-graphs are exactly the configurations in which no subspace is a binomial configuration of one-step-smaller size n . 2 n+1 n+1 A binomial 2 n−1 3 3 -configuration with two Kn-subgraphs can be considered as an abstract scheme of a perspective of two n-simplices (Prop. 3.6 ). Generally, the structure of the intersection points of Kn-subgraphs of a n+1 n+1 2 n−1 3 3 -configuration is isomorphic to a generalized Desargues config- uration (Prop. 3.9). In the paper we do not intend to give a detailed analysis of the internal struc- ture of binomial configurations which contain a prescribed number of their maximal Kn-subgraphs. We give only a technique (a procedure) to construct a binomial config- uration which contains at least the given number of Kn-subgraphs: Theorem 3.12, and we prove that for each admissible integer m there does exist a binomial configuration which freely contains mKn-subgraphs. Some remarks are also made which show that, surprisingly, binomial configurations with ‘many’ maximal complete subgraphs (the maximal admissible number, the maximal number reduced by 2, and the maximal number reduced by 3) are the configurations of some well known classes. 2 Definitions Recall that the term combinatorial configuration (or simply a configuration)isusu- ally (cf. e.g. [9,22]) used for a finite incidence point-line structure provided that two different points are incident with at most one line and the line size and point rank are constant. Speaking more precisely, a (vr b )-configuration is a combinatorial config- uration with v points and b lines such that there are r lines through each point, and there are points on each line. A partial Steiner triple system (a PSTS, in short) is a configuration whose each line contains exactly three points. In what follows we shall pay special attention to so called binomial configura- tions (more precisely: binomial partial Steiner triple systems, BSTS, in short) i.e. to n n ≥ 2 n−2 3 3 -configurations with arbitrary integer n 2. The class of the configura- tions with these parameters will be denoted by Bn. ℘ Let k be a positive integer and X aset;wewrite k(X) forthesetofallk-subsets of X. The incidence structure ℘ ℘ Gk(X) := k(X), k+1(X), ⊂ will be called a combinatorial Grassmannian (cf. [19]). If |X|=n then G2(X) is a Bn-configuration, so it is a BSTS. As the structure G2(X) is(uptoanisomor- phism) uniquely determined by the cardinality |X| in what follows we frequently write G2(|X|) instead of G2(X). Recall that G2(4) is the Veblen (the Pasch) configuration and G2(5) is the Desargues Configuration (see e.g. [17,19]). Generally, every structure G2(n), n ≥ 5 will be called a generalized Desargues configuration (cf. [4,5,21]). 123 Graphs and Combinatorics Let X be a nonempty set with |X|=n. A nondirected loopless graph defined on ℘ X (we say simply: a graph) is a structure of the form X, P with P ⊂ 2(X).We ℘ write K X =X, 2(X) for the complete graph with the vertices X;thetermKn is used for an arbitrary graph K X where |X|=n. We say that a configuration N =X, G is contained in a configuration M =S, L if – X ⊂ S. – each line L ∈ G extends (uniquely) to some L ∈ L, and – distinct lines of N extend to distinct lines of M. N is l-closed (is a subspace of M) if each line of M that crosses X in at least two points is an extension of a line in G. In that case we also say that N is regularly contained in M. Frequently, with a slight abuse of language, we refer to lines of the form L (L ∈ G) as to sides of N. N is p-closed if its sides do no intersect outside N i.e. when the following holds: L1, L2 ∈ G, L1 = L2, p ∈ L1 ∩ L2 ⇒ p ∈ X A graph contained and p-closed in M is said to be freely contained in M.